Cook the spaghetti in salted boiling water until al dente. Save 1/4 cup pasta water, then drain.
Melt the butter in a large skillet over medium-low heat. Add the garlic and cook for 30 seconds.
Pour in the heavy cream and milk. Stir in the cream cheese, Velveeta, garlic powder, Italian seasoning, salt, and black pepper. Cook gently until smooth.
Add the mozzarella and Parmesan. Stir until fully melted and the sauce is thick, creamy, and glossy.
Add the cooked spaghetti and toss until every strand is coated. Add a splash of reserved pasta water if needed to loosen the sauce slightly.
Sprinkle with parsley and serve.